1. Field of the Invention
Aspects of the present invention generally relate to a skew correction apparatus for correcting a skew of a sheet and an image forming apparatus including the skew correction apparatus.
2. Description of the Related Art
Known image forming apparatuses for forming an image on a sheet are provided with, to form the image without skew with respect to the sheet, a skew correction apparatus for correcting the skew of sheet conveyed to an image formation unit.
Generally, in the skew correction apparatuses, on a conveyance path between a sheet feeding unit for feeding a sheet and an image forming unit, registration rollers and a shutter are provided upstream of the image forming unit. The sheet skew correction operation with the registration roller pair and the shutter will be described with reference to Japanese Patent Application Laid-Open No. 6-345294. At the upstream side of the registration roller pair and the shutter, a roller pair (hereinafter, referred to as an upstream roller pair) is provided. The upstream roller pair nips and conveys the sheet fed from the sheet feeding unit toward the registration roller pair and the shutter. The leading edge of the sheet is pressed against the contact portion such as the registration rollers and the shutter to correct the skew of the leading edge portion of the sheet.
In the skew correction apparatus discussed in Japanese Patent Application Laid-Open No. 6-345294, an upstream roller pair nips and conveys the sheet such that the leading edge of the sheet comes in contact with the nip portion of the registration roller pair to form a loop. By the operation, the leading edge of the sheet is arranged along the nip portion and the skew of the sheet is corrected. In other words, between the registration roller pair and the upstream roller pair, a loop of a desired size of the sheet is formed such that the leading edge of the sheet and the shaft line of the registration roller pair become parallel to each other, and the skew of the leading edge portion of the sheet is corrected.
However, nowadays, image forming apparatuses are desirable to perform image formation on various types of sheet having different size, basis weight, or glossiness. To perform the skew correction on such various types of sheet, the known skew correction apparatuses have room for improvement.
In correcting a skew of sheet by making the leading edge of the sheet conveyed with the upper roller pair come in contact with the contact portion to form a loop of the sheet, a force to return to the original state (plane state) (hereinafter, referred to as loop reaction force) acts on the sheet having the formed loop. In a case of a sheet having a relatively large basis weight, the loop reaction force is larger. As a result, before a loop of a desired size is formed such that the leading edge of the sheet become parallel to the shaft line of the registration roller pair, the sheet slips at the upper roller pair.
While the sheet is slipping at the upper roller pair, if the sheet turns such that the leading edge of the sheet becomes parallel to the shaft line of the registration roller pair, the skew of the sheet is corrected. However, if the sheet is nipped with the upper roller pair and a roller pair located further upstream of the upper roller pair, the turning performance of the sheet is affected. As a result of the effect on the turning performance of the sheet, the leading edge of the sheet remains skewed with respect to the shaft line of the registration roller pair. This decreases the sheet skew correction performance.
The decrease in the sheet skew correction performance causes improper image formation on the sheet, resulting in image failure.